Note: Descriptions are shown in the official language in which they were submitted.
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ARC FAULT DETECTOR APPARATUS, MEANS AND SYSTEM
TECHNICAL FIET-B
The present invention relates, in general, to arc
fault detectors and, more specifically, to sputtering arc
fault detectors using high pass filtering techniques.
BACKGROUND O THE TNV ,NTTON
There are various conditions that may cause an arc
fault. Corroded, worn or aged wiring or insulation, worn
power cords, old wall outlets with insufficient contact
pressure, electrical stress from repeated overloading, etc.,
may result in an arc fault. These conditions may damage the
insulation of the wiring and create excessive heating
temperatures. Arc faults may result in a fire depending on
various conditions, such as if combustible materials are in
close proximity.
There are also various conditions that may result
in a false arc fault. For example, the occurrence of an arc
fault in one branch circuit of a power distribution system
may generate a false arc in another branch circuit. As a
result, circuit breakers in more than one branch circuit may
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erroneously trip. Another example is a relatively noisy load
such as an electric drill creating a high frequency
disturbance in the circuit, which may appear to be an arc
fault.
There are two types of arc faults that may occur in
a home. A first type is a high-energy arc that may be
related to high current faults; a second type is a low
current arc that may be related to persistent momentary
contact of electrical conductors. The first type may result
from inadvertent connection between a line conductor and
neutral conductor or a line conductor and ground. The first
type may draw current that is above the rated capacity of the
circuit, arcing as the contacts are physically joined.
The other type of arc fault, the momentary contact
of electrical conductors, may be considered more problematic.
Since the current in the arc may be limited to less than the
trip rating of an associated circuit breaker, such arcs may
become persistent without observation and may result in
certain conditions. Contact arcs may be caused by springs in
switches that become worn which, in turn, may reduce the
forces that hold electrical contacts together. As the
electrical contacts heat and cool down, the conductors may
touch and separate repeatedly, thereby possibly creating arcs
known as "sputtering arcs."
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Contact arcs or sputtering arcs may also be
observed in contacts which are made from different materials.
For example, aluminum wiring which contacts copper wiring may
oxidize at the contact points. In this case a non-conductive
layer may build up over time between the contact points and
arcing may result. Sputtering arcs may also be observed in
extension cords having insufficient current carrying
capacity. As the plug is heated by resistance heating,
insulating materials around the contacts may eventually melt
and flow into the contact area, preventing proper contact
from being made. The current in the conductors may produce
magnetic repulsion forces which may push the conductors
apart, resulting in an arc. The arc may be extinguished as
the current passes through zero. Mechanical or electro-
static forces may bring the conductors back into contact, and
the cycle may be repeated.
It is believed that various circuit breakers are
not specifically designed to guard against sputtering arcs.
Special purpose detectors have been designed to detect
sputtering arcs and, when detected, trip the circuit
breakers. Some detectors are believed to depend on the
sputtering arcs exceeding a predetermined current or voltage
threshold before tripping the circuit breaker; other
detectors are believed to depend on the sputtering arcs
having a specific high frequency signature. Still other
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detectors are believed to depend on the sputtering arcs
producing a wideband high frequency noise ranging from 10 kHz
to 1 GHz while the arc is conducting current. These
detectors may require that no noise be produced while the arc
is not conducting current, i.e., during the gaps between arc
conduction. These detectors use various processing
techniques to analyze the repetitive patterns in the noise.
It is believed that a need still exists for a
sputtering arc fault detector that does not depend on the
frequency content of the noise generated by the arc or on
relatively involved analysis techniques to determine the arc
signature characteristics.
SUMMARY OF TT 4F NTTON
To meet this and other needs, and in view of its
purposes, the present invention provides a fault detector
that monitors rapid transition of current flow in conjunction
with current magnitude. In one embodiment, a sputtering arc
fault detector is provided for a system having an electrical
conductor carrying current to a load. The sputtering arc
fault detector includes a current monitor coupled to the
conductor for generating a variable signal responsive to
behavior of the current in the conductor. A level detector is
coupled to the monitor and generates a first pulse when the
variable signal exceeds a first level. A step detector is
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coupled to the monitor and is responsive to rapid step
increases of the variable signal. The step detector generates
a second pulse when the variable signal exceeds a second
level. An arc verifier, which is coupled to the level
detector and the step detector, combines the first and second
pulses, and generates a fault signal when the combined pulses
exceed a third level. Furthermore, the arc verifier includes
a transistor having first and second terminals that define a
principal conduction path and a third terminal that controls
conduction through the path. The first terminal is coupled to
the level detector, the third terminal is coupled to the step
detector, and the second terminal provides an output signal
which represents a combination of the first and second
pulses. A capacitor is coupled to the second terminal, and
charges in response to the combined pulses. A comparator is
coupled to receive and compare the charge to the third level.
The fault signal is generated by the comparator when the
charge exceeds the third level. The capacitor charges at a
first rate and discharges at a second rate which is slower
than the first rate. The comparator includes a hysteresis
circuit which holds the fault signal when the charge exceeds
the third level.
Another object of the invention is to provide a
sputtering arc fault detector for a system having an
electrical conductor carrying current to a load, said
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sputtering arc fault detector comprising: a current monitor
coupled to the conductor for generating a variable signal
responsive to behavior of said current in said conductor; a
level detector coupled to said monitor and generating a first
pulse when said variable signal exceeds a first level; a step
detector coupled to said monitor and responsive to rapid step
increases of said variable signal, said step detector
generating a second pulse when said variable signal exceeds a
second level; and an arc verifier coupled to said level
detector and said step detector for combining said first
pulse and said second pulse, said arc verifier generating a
fault signal when said combined pulses exceed a third level.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
arc verifier includes a transistor having first and second
terminals that define a principal conduction path and a third
terminal that controls conduction through said path, said
first terminal being coupled to said level detector, said
third terminal being coupled to said step detector, and said
second terminal providing an output signal which represents a
combination of said first and second pulses, a capacitor
coupled to said second terminal, said capacitor charging
responsive to said combined pulses, and a comparator coupled
to receive and compare said charge to said third level,
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wherein said fault signal is generated by said comparator
when said charge exceeds said third level.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
capacitor charges at a first rate and discharges at a second
rate which is slower than said first rate.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
comparator includes a hysteresis circuit, and said fault
signal is held when said charge exceeds said third level.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
hysteresis circuit includes: a resistor coupled to an input
of said comparator for modifying said third level, and a
transistor having fourth and fifth terminals that define a
principal conduction path and a sixth terminal that controls
conduction through said path, said fourth and fifth terminals
being coupled to said resistor, and said sixth terminal being
coupled to an output of said comparator, wherein said third
level is modified to a lower potential level when said fault
signal is generated.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
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step detector includes a high pass filter coupled to said
current monitor, and a first window comparator having an
input side coupled to said high pass filter, and an output
side coupled to said arc verifier, wherein said high pass
filter provides an edge signal to said input side when said
rapid step increase occurs, and said first window comparator
provides said second pulse when said edge signal exceeds said
second level.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
level detector includes a second window comparator having an
input side coupled to said current monitor, and an output
side coupled to said arc verifier, wherein said second window
comparator provides said first pulse when said variable
signal exceeds said first level.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
monitor includes a current transformer coupled to said
conductor, a node connecting said monitor to said level
detector and step detector, and a buffer between said current
transformer and said node.
Another object of the invention is to provide the
sputtering arc fault detector described above further
including a circuit breaker for interrupting current flow to
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said load, a SCR circuit coupled to said arc verifier, and a
solenoid coupled between said SCR circuit and said circuit
breaker, wherein said SCR circuit is conditioned to trip said
circuit breaker when said fault signal is generated.
Another object of the invention is to provide the
sputtering arc fault detector described above wherein said
electrical conductor includes a line conductor and a neutral
conductor coupled across said load, said sputtering arc fault
detector further including a differential current monitor
coupled to said line and neutral conductors for generating a
differential signal responsive to behavior of said current in
said line and neutral conductors, a fault level detector
coupled to said differential current monitor which generates
a ground fault signal when said differential signal exceeds a
fourth level, and a further SCR circuit coupled between said
fault level detector and said solenoid, wherein said further
SCR circuit is conditioned to trip said circuit breaker when
said ground fault signal exceeds said fourth level.
A further object of the invention is to provide a
method for detecting a sputtering arc fault in a system
having an electrical conductor carrying current to a load,
said method comprising the steps of: a) monitoring the
current of the conductor and generating a variable signal
responsive to behavior of said current in said conductor; b)
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measuring said variable signal in magnitude and generating a
first pulse when said magnitude exceeds a first level; c)
detecting a rapid step increase of said variable signal, and
generating a second pulse when said step increase exceeds a
second level; and d) combining said first and second pulses
to generate a third pulse when said third pulse exceeds a
third level.
A further object of the invention is to provide the
method described above wherein step (d) includes charging a
capacitor responsive to said third pulse, comparing said
charge in magnitude to said third level, and generating said
fault signal when said magnitude exceeds said third level.
A further object of the invention is to provide the
method described above wherein said charging is at a first
rate and includes discharging said capacitor at a second rate
which is slower than said first rate.
A further object of the invention is to provide the
method described above including the step of holding said
fault signal when said charge exceeds said third level.
A further object of the invention is to provide the
method described above wherein said holding includes the step
of modifying said third level to a lower magnitude when said
fault signal is generated.
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A further object of the invention is to provide the
method described above wherein step (c) includes the step of
high pass filtering said variable signal, and generating said
second pulse in response to said high pass filtered variable
signal.
Still another object of the invention is to provide
a sputtering arc fault detector for a system having an
electrical conductor carrying current to a load, said
sputtering arc fault detector comprising: a current monitor
coupled to said conductor for generating a variable signal
responsive to behavior of said current in said conductor; a
level detector coupled to said monitor and generating a first
pulse when said variable signal exceeds a first level; a high
pass filter having one end coupled to said current monitor
and a further end responsive to a rapid step increase of said
variable signal; a step detector coupled to said further end
and generating a second pulse when said rapid step increase
exceeds a second level; a transistor having first and second
terminals that define a principal conduction path and a third
terminal that controls conduction through said path, said
first terminal being coupled to said level detector, said
third terminal being coupled to said step detector, and said
second terminal providing an output signal which represents a
combination of said first and second pulses; a capacitor
coupled to said second terminal, said capacitor charging
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responsive to said combined pulses; and a comparator coupled
to receive and compare said charge to a third level, said
comparator generating a fault signal when said charge exceeds
said third level.
Still another object of the invention is to provide
a sputtering arc fault detector for a system having an
electrical conductor carrying current to a load, said
sputtering arc fault detector comprising: a current monitor
coupled to said conductor for generating a variable signal
responsive to behavior of said current in said conductor; a
level detector coupled to said monitor and generating a first
pulse when said variable signal exceeds a first level; a high
pass filter having one end coupled to said current monitor
and a further end responsive to a rapid step increase of said
variable signal; a step detector coupled to said further end
and generating a second pulse when said rapid step increase
exceeds a second level; a transistor having first and second
terminals that define a principal conduction path and a third
terminal that controls conduction through said path, said
first terminal being coupled to said level detector, said
third terminal being coupled to said step detector, and said
second terminal providing an output signal which represents a
combination of said first and second pulses; a capacitor
coupled to said second terminal, said capacitor charging
responsive to said combined pulses; a comparator coupled to
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receive and compare said charge to a third level, said
comparator generating a fault signal when said charge
exceeds said third level; a circuit breaker for interrupting
current flow to said load; a SCR circuit coupled to said
comparator; and a solenoid coupled between said SCR circuit
and said circuit breaker; wherein said SCR circuit is
conditioned to trip said circuit breaker when said fault
signal is generated.
In accordance with one aspect of this invention,
there is provided a sputtering arc fault detector for a
system having an electrical conductor carrying current to a
load, said sputtering arc fault detector comprising: a
current monitor coupled to the conductor for generating a
variable signal responsive to behaviour of said current in
said conductor, a level detector coupled to said monitor and
generating a first pulse when said variable signal exceeds a
first level, a step detector coupled to said monitor and
responsive to rapid step increases of said variable signal,
said step detector generating a second pulse when said
variable signal exceeds a second level, and an arc verifier
coupled to said level detector and said step detector for
combining said first pulse and said second pulse, said arc
verifier generating a fault signal when said combined pulses
exceed a third level.
In accordance with another aspect of this
invention, there is provided a sputtering arc fault detector
for a system having an electrical conductor carrying current
to a load, said sputtering arc fault detector comprising: a
current monitor coupled to said conductor for generating a
variable signal responsive to behaviour of said current in
said conductor, a level detector coupled to said monitor and
generating a first pulse when said variable signal exceeds a
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first level, a high pass filter having one end coupled to
said current monitor and a further end responsive to a rapid
step increase of said variable signal, a step detector
coupled to said further end and generating a second pulse
when said rapid step increase exceeds a second level, a
transistor having first and second terminals that define a
principal conduction path and a third terminal that controls
conduction through said path, said first terminal being
coupled to said level detector, said third terminal being
coupled to said step detector, and said second terminal
providing an output signal which represents a combination of
said first and second pulses, a capacitor coupled to said
second terminal, said capacitor charging responsive to said
combined pulses, and a comparator coupled to receive and
compare said charge to a third level, said comparator
generating a fault signal when said charge exceeds said
third level.
In accordance with a further aspect of this
invention, there is provided a sputtering arc fault detector
for a system having an electrical conductor carrying current
to a load, said sputtering arc fault detector comprising: a
current monitor coupled to said conductor for generating a
variable signal responsive to behavior of said current in
said conductor, a level detector coupled to said monitor and
generating a first pulse when said variable signal exceeds a
first level, a high pass filter having one end coupled to
said current monitor and a further end responsive to a rapid
step increase of said variable signal, a step detector
coupled to said further end and generating a second pulse
when said rapid step increase exceeds a second level, a
transistor having first and second terminals that define a
principal conduction path and a third terminal that controls
conduction through said path, said first terminal being
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coupled to said level detector, said third terminal being
coupled to said step detector, and said second terminal
providing an output signal which represents a combination of
said first and second pulses, a capacitor coupled to said
second terminal, said capacitor charging responsive to said
combined pulses, a comparator coupled to receive and compare
said charge to a third level, said comparator generating a
fault signal when said charge exceeds said third level, a
circuit breaker for interrupting current flow to said load,
a SCR circuit coupled to said comparator, and a solenoid
coupled between said SCR circuit and said circuit breaker,
wherein said SCR circuit is conditioned to trip said circuit
breaker when said fault signal is generated.
In accordance with a further aspect of this
invention, there is provided a method for detecting a
sputtering arc fault in a system having an electrical
conductor carrying current to a load, said method comprising
the steps of: (a) monitoring the current of the conductor
and generating a variable signal responsive to behavior of
said current in said conductor; (b) measuring said variable
signal in magnitude and generating a first pulse when said
magnitude exceeds a first level; (c) detecting a relatively
rapid step increase of said variable signal, indicating a
relatively rapid transition of current flow of said current,
and generating a second pulse when said step increase
exceeds a second level; (d) combining said first and second
pulses to produce a third pulse, wherein when said third
pulse exceeds a third level said third pulse indicates said
relatively rapid transition of said current flow in
conjunction with said magnitude to thereby indicate said
sputtering arc fault in said system.
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It is understood that the foregoing general
description and the following detailed description are
exemplary, but are not restrictive of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is best understood from the
following detailed description when read in connection with
the accompanying drawings. Included in the drawings are the
following figures:
FIG. 1 is a schematic circuit diagram, partially
in block diagram form, of an arc fault detector in
accordance with the invention.
FIG. 2 is a schematic circuit diagram of portions
of the arc fault detector of FIG. 1.
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FIGS. 3A-3D are waveform diagrams illustrating the
voltages produced at various portions of the arc fault
detector of FIGS. 1 and 2.
DETAILED DESCRIPTION OF T1 4F
Shown in FIG. 1 is an exemplary embodiment of a
fault detector 10. As shown, fault detector 10 combines an
arc fault interrupt (AFI) sensor 66 with a differential
sensor 90 to monitor the current drawn by an electric load
(not shown). Sensor 90 is coupled to line conductor 12 and
neutral conductor 14, and sensor 66 is coupled to neutral
conductor 14. As described below, neutral current monitor
64, current level detector 58, current step detector 62 and
arc verifier 48, in combination, are designed to protect
against certain sputtering arc faults occurring between the
line conductor and neutral conductor. As is also described
below, differential current monitor 68 and ground fault level
detector 54, in combination, are designed to protect against
certain over currents drawn by the load and line to ground
faults occurring in the electric system.
Although not shown in FIG. 1, it will be understood
that the electric system may have a voltage source from a
power utility company supplying 120 volts between the line
conductor and the neutral conductor, all at, for example, 60
Hz. The invention is not limited to such example, and may be
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applied to any ac system operating at any voltage and
frequency, including, for example, 240 volts, 480 volts and
660 volts, and 50, 60 and 400 Hz.
If a sputtering arc condition is determined to be
present, arc verifier output 44 goes high to turn on SCR
circuit 40. Turning on SCR circuit 40 provides current to
energize solenoid 36, with current drawn from the line and
neutral conductors. Solenoid 36, when actuated, trips the
circuit breaker to open contacts 16 using trip mechanism 18.
It will also be appreciated that AFI sensor 66 is a torroidal
current transformer having a secondary transformer sensing
current flowing through the neutral conductor. Differential
sensor 90 is also a torroidal current transformer which
senses current flowing through neutral conductor 14 and line
conductor 12, both of which are coupled through the center of
the torus. In this configuration, only differential current
between the conductors is sensed.
If a ground fault is determined to be present,
ground fault level detector output 46 goes high to turn on
SCR circuit 42. Turning on SCR circuit 42 also provides
current to energize solenoid 36 and open contacts 16. Thus,
SCR circuit 40 and SCR circuit 42 are logically OR-ed at
junction 38 to operate solenoid 36 and trip mechanism 18. It
will be appreciated that SCR circuit 40 and SCR circuit 42
are protected from surges on the line and neutral conductors
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by metal oxide varistor (MOV) 32 connected across line
conductor 12 and neutral conductor 14. Furthermore, diode 34
is intended to suppress surges across solenoid 36.
Also shown in FIG. 1 is regulated power supply 28
which is connected between line conductor 12 and ground
terminal 30. Regulated power supply 28 rectifies 60 Hz ac
line voltage into a steady state dc voltage of Vcc, which is
provided at node 20. Using resistive voltage divider
networks, such as resistor 24 and resistor 92, shown
connected in series between Vcc node 20 and ground terminal
30, other dc voltages such as VDD may be provided at node 26.
As described below, Vcc and VDD voltages are provided to
various circuits of fault detector 10 for operating purposes.
Detection of sputtering arc faults, which is
accomplished by the combination of neutral current monitor
64, current level detector 58, current step detector 62 and
arc verifier 48, is now described by reference to FIGS. 1 and
2. (Like numerals refer to like elements.) Neutral current
monitor 64 includes operational amplifier 110 which has a
gain dependent on the resistive values of resistor 108 and
resistor 104. In the exemplary embodiment, the gain of
operational amplifier 110 is set to unity. As a result,
voltage swings across resistor 102 may be seen with unity
gain at the output of operational amplifier 110, or at node
60. Since the non-inverting side of operational amplifier
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110 is connected to VDD at node 26, the output of operational
amplifier 110 has a dc level of VDD. In the exemplary
embodiment, VDD is one-half of Vcc. In essence, operational
amplifier 110 is a buffer which monitors the current
fluctuation in AFI sensor 66, and provides the same signal
fluctuation at output node 60, but shifted in dc level.
Thus, as an example, the signal output at node 60 may be a 60
Hz sinusoidal signal centered at a VDD level. The peak-to-
peak level of the signal will, of course, depend on the
current magnitude sensed by AFI sensor 66. Finally,
capacitor 106 presents a low impedance to high frequency
current fluctuations that may be sensed by AFI sensor 66, for
example high frequency fluctuations due to sputtering. In
this manner, high frequency signals, above 60 Hz
fluctuation, are amplified by operational amplifier 110 and
transferred to the output at node 60.
Current level detector 58 is now described.
Current level detector 58 includes a pair of operational
amplifiers, comparator 116 and comparator 120. The output of
neutral current monitor 64 is fed to the pair of comparators
that form a window comparator. Specifically, the output at
node 60 is applied to the non-inverting input of the
comparator 116 and the inverting input of comparator 120.
The other inputs of the comparators receive two different
reference voltages which set the limits of the window, i.e.,
the only signals that pass through the window comparator are
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those that are greater than the reference voltage set by the
inventing input of comparator 116 and those that are less
than the reference voltage set by the non-inverting input of
comparator 120. The reference voltage of comparator 116 is
set by the voltage divider network of resistor 114 and
resistor 126; the reference voltage of comparator 120 is set
by the voltage divider network of resistor 112 and resistor
124. In the exemplary embodiment, both reference voltages
are set to the same magnitude but with opposite polarity
relative to VDD.
Finally, the signals that pass through comparators
116 and 120 are logically OR-ed by diodes 118 and 122 to
provide a combined output at node 50. As an example of the
operation of current level detector 58 reference is now made
to FIGS. 3A and 3B. As shown, a low amplitude 60 Hz sinusoid
250 and a high amplitude 60 Hz sinusoid 252 are output from
neutral current monitor 64. Moreover, current waveforms 254
and 256 may be indicative of sputtering arcs sensed in the
neutral conductor. In response to the signals shown in FIG.
3A, current level detector 58 provides (1) pulse output 258
during the positive portion of waveform 254 which exceeds the
reference voltage, VREF, and (2) pulse output 260 during the
negative portion of waveform 256 which is below the reference
voltage, VDD-VREF. Since a wider portion of waveform 252 is
above VREF than the portion of waveform 254 or 256
is wider than 258, or pulse,260. Thus, current level detector
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58 provides an output at node 50 that is indicative of the
period of time that the magnitude of the current in neutral
conductor 14 exceeds a predetermined threshold.
Current step detector 62 is now described. Current
step detector 62 includes another pair of operational
amplifiers, comparator 140 and comparator 142. The output
terminal of neutral current monitor 64 is ac-coupled through
capacitor 130 to the pair of comparators 140 and 142 that
form a second window comparator. Capacitor 130 and resistor
132 form a high-pass filter. The second window comparator
operates in a fashion similar to the window comparator
described above. The reference voltage of comparator 140 is
set by the voltage divider network of resistor 134 and
resistor 144; the reference voltage of comparator 142 is set
by the voltage divider network of resistor 136 and resistor
146. In the exemplary embodiment both reference voltages are
set to the same magnitude but opposite polarities relative to
VDD. The signals that pass through comparators 140 and 142
are logically OR-ed by way of resistor 138 to provide a
combined output at node 52.
As an example of the operation of current step
detector 62, reference is now made to FIGS. 3A, 3C and 3D.
As shown, waveforms 254 and 256 are passed by the high-pass
filter to the input side of the window comparator to capture
the relatively fast transitions of waveforms 254 and 256
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(shown in FIG. 3C as edge signals 262 and 264) For every
leading edge that exceeds the positive or negative reference
voltage of the window comparator an output is provided at
node 52. This may be seen in FIG. 3D as a series of
multiple, negative-going pulses 266 and 268. (The pulses
transition from a high-level to a low-level.) Thus, current
step detector 62 provides an output signal at node 52 that is
indicative of the period of time that the current in neutral
conductor 14 experiences fast rising leading edges which
exceed a predetermined threshold.
Lastly, arc verifier 48 is described. The output
signal of current level detector 58 at node 50 is logically
AND-ed with the output of current step detector 62 at node 52
by PNP transistor 152. It will be appreciated that
transistor 152 is turned-on, when both a positive signal at
node 50 and a less positive signal at node 52 exist. The
logically AND-ed output signal of transistor 152 quickly
charges capacitor 160 through resistor 154. The capacitor
slowly discharges through resistor 162. In the exemplary
embodiment, resistor 154 is 2.43KQ, resistor 162 is 750KQ
and capacitor 160 is 0.1 F. Thus, an analog integrator is
formed for the number of half-cycles of arcing detected at
neutral conductor 14. As more half-cycles of arcing are
detected, capacitor 160 charges to a higher magnitude and
discharges slowly when no arcing is detected.
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The voltage across capacitor 160 is provided as an
input to the non-inverting side of comparator 156.
Establishing a voltage reference for comparator 156 is the
resistive network formed by resistors 150, 164 and 168. In
the exemplary embodiment, resistors 150, 164 and 168 are
47.5KQ, 10.OKQ and 15.OKQ, respectively. When the voltage
across capacitor 160 exceeds a predetermined level,
comparator 156 sends an output signal, by way of resistor
158, to node 44 which fires SCR circuit 40. Furthermore,
comparator 156 sends an output signal, by way of resistor
166, which turns-on NPN transistor 170, thereby shorting
resistor 168 to a source of reference potential (e.g.
ground). In turn, the voltage reference for comparator 156
is now lowered. Thus, a hysteresis circuit is provided to
keep SCR circuit 40 in a fired-state. Thus, the latched or
holding condition exists, until the voltage across capacitor
160 decays to a value less than the new threshold level.
In summary, arc verifier 48 provides an output to
fire SCR circuit 40 only when four conditions have occurred.
First, the magnitude of the current in neutral conductor 14
exceeds a first predetermined threshold. Second, the leading
edge of the current waveform is sharp enough to pass through
the high pass filter. Third, the magnitude of the leading
edge of the current waveform exceeds a second predetermined
threshold. Fourth, a sufficient number of these high
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magnitude leading edges have occurred before the SCR circuit
40 fires.
Still referring to FIGS. 1 and 2, differential
current monitor 68 and ground fault level detector 54 are now
described. The signal sensed by differential sensor 90 is
placed across resistor 172 and is generally a relatively low-
level voltage. A clipping network, formed from oppositely
connected diodes 174 and 176, protects amplifier 188 from
voltage excursions above a predetermined level, for example
600 mV. For further protection, coupling capacitors 178, 180
and 182 are connected as shown to divert any relatively high
frequency signals present at the input to amplifier 188. The
differential signal across resistor 172 is amplified by
amplifier 188. In the exemplary embodiment, input resistors
184 and 186 are each 3.01KS2, where as output resistors 192
and 194 are 301KQ and 30.1Kg, respectively, resulting in an
amplification gain of 100 times. It will be appreciated that
capacitor 190 reduces the high frequency gain of amplifier
188.
The amplified differential signal from differential
current monitor 68 is provided to ground fault level detector
54, the latter having an input node 56 and an output node 46.
Ground fault level detector 54 includes a pair of comparators
208 and 210 forming a window comparator. This window
comparator operates in a manner substantially similar to the
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window comparators previously described. The voltage
reference for comparator 208 is determined by the resistive
network of resistors 202 and 206; the voltage reference for
comparator 210 is determined by the resistive network of
resistors 200 and 204. In the exemplary embodiment, the
voltage references are set to have the same magnitude but
opposite polarity relative to VDD. Finally, the output from
comparator 208 and the output from comparator 210 are
logically OR-ed by diodes 212 and 214, and provided through
resistor 216 to fire SCR circuit 42. It will be appreciated
that firing of SCR circuit 42 is independent of firing SCR
circuit 40.
The following table provides values for the
exemplary embodiment shown in FIG. 2.
Resistor Value (K-ohms)
102 10.0
104 10.0
108 10.0
112 34.8
114 20.0
132 10.0
124 20.0
126 34.8
134 75.0
136 56.2
138 22.1
150 47.5
154 2.43
144 56.2
146 75.0
162 5000.0
164 10.0
166 75.0
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168 15.0
166 75.0
158 10.0
172 3.01
184 3.01
186 3.01
192 301.0
194 30.1
200 47.5
202 30.1
204 30.1
206 47.5
216 10.0
Capacitor Value (pF)
106 220
130 470
160 10
178 470
180 470
182 2200
190 220
218 220
Voltage Value
VCC 9 . Ov
VDD 4.5v
Although illustrated and described herein with
reference to certain specific embodiments, the present
invention is nevertheless not intended to be limited to the
details shown. Rather, various modifications may be made in
the details within the proper scope of the claims and without
departing from the spirit of the invention. It will be
understood, for example, that the present invention is not
limited to only a fault detector for a household load, which
carries 60 Hz current from a 120 volt supply. Rather this
invention may be extended to commercial and industrial loads
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carrying current at various frequencies from various voltage
supplies.
Furthermore, although the exemplary embodiment in
FIG. 2 shows the use of bipolar transistors, it will be
understood that MOSFETs may also be used. The description
provided before is the same except that the gate, source and
drain terminals of the MOSFET correspond to the base, emitter
and collector terminals of the bipolar transistor. Of
course, any other suitably appropriate switching device may
also be used.
